Dynamic Random Access Memory chips (DRAMs) are widely used among semiconductor memory devices because of their high density and low cost. The basic operation of a DRAM is to record data by storing bits as the presence or absence of electrical charge in memory cell capacitors. Silicon oxide film or silicon nitride film is often used as the dielectric film in the memory cell capacitor of the DRAM, but if the memory data is to be nonvolatile, ferroelectric film made of barium titanate and other materials can be used instead.
Other than being used independently, semiconductor memory devices are often used as the memory area of micro computers. These conventional semiconductor memory devices having ferroelectric capacitors are manufactured through the following process:
First, oxide film for separating elements is formed on a silicon substrate, then a Metal Oxide Semiconductor (MOS) transistor comprising diffusion layers, a gate insulating film and a gate electrode is formed. Next, a ferroelectric capacitor fabricated by three layers, namely a lower electrode, ferroelectric film and an upper electrode deposited in this order from the bottom, is formed on the oxide film used for separating elements, and covering the whole silicon substrate is an interlayer insulating film comprising silicon oxide film.
Second, first contact holes reaching to the diffusion layers and second contact holes reaching both to the lower and upper electrodes of the ferroelectric capacitor are formed on the interlayer insulating film. Then heat treatment is provided to the substrate including the above process in an hydrogen atmosphere. Through the first contact holes, metal interconnection contacting the diffusion layers is formed. Also through the second contact holes, metal interconnection contacting both the upper and lower electrodes is formed.
In general, at the manufacturing process of MOS transistors, surface state density on interface between a silicon substrate and the insulating film, gate insulating film among others, formed thereon increases, which degrades transistor characteristics. Therefore, in order to lower the surface state density, the heat treatment in hydrogen atmosphere should be provided without fail. However, when a ferroelectric oxide is used as a dielectric film in a memory cell capacitor, the ferroelectric oxide absorbs hydrogen through the second contact holes during the heat treatment process in hydrogen, which degrades electrical characteristics.